Currently, most commercial photocurable dental restorative resins are based on dimethacrylates and the reaction mechanism is through chain-growth free radical polymerization. Existing dimethacrylate systems are popular for fillings and other dental prostheses because of their esthetic merit and “cure-on-command” feature. These formulations have resulted in significant advancements in the field of dentistry.
Such dental restorative materials are often mixed with 45 to 85% by weight (wt %) silanized filler compounds such as barium, strontium, zirconia silicate and/or amorphous silica to match the color and opacity to a particular use or tooth. The filler is typically in the form of particles with a size ranging from 0.01 to 5.0 micrometers.
The photocurable restorative materials are often sold in separate syringes or single-dose capsules of different shades. If provided in a syringe, the user dispenses (by pressing a plunger or turning a screw adapted plunger on the syringe) the necessary amount of restorative material from the syringe. Then, the material is placed directly into the cavity, mold, or location of use. If provided as a single-dose capsule, the capsule is placed into a dispensing device that can dispense the material directly into the cavity, mold, and such. After the restorative material is placed, it is photopolymerized or cured by exposing the restorative material to the appropriate light source. The resulting cured polymer may then be finished or polished as necessary with appropriate tools. Such dental restoratives can be used for direct anterior and posterior restorations, core build-ups, splinting and indirect restorations including inlays, onlays and veneers.
Although easy to use, these dimethacrylate systems have several drawbacks and there are a number of properties of the resin chemistry that, if improved upon, would increase the performance, longevity and biocompatibility of composite restorations (Sakaguchi et al., 2005, Dent. Mat. 21:43-46; Dauvillier et al., 2001, J. Biomed. Mat. Res. 58(1):16-26, 2001; Dauvillier et al., 2000, J. Dent. Res. 79(3):818-823; Yourtee et al., 1997, In Vitro Tox. 10:245-251). The most significant shortcomings of methacrylate-based resins are high volumetric shrinkage (Ferracane, 2005, Dent. Mat. 21:36-42), high polymerization stress (Braga et al., 2005, Dent. Mat. 21:962-970; Lu et al., 2005, Dent. Mat., 21(12):1129-1136; Braga and Ferracane, 2002, J. Den.1 Res. 81:114-118) and low functional group conversion (Darmani and Al-Hiyasat, 2006, Dent. Mat. 22:353-358; Sasaki et al., 2005, J. Mat. Sci.: Mat. Med. 16:297-300; Pulgar et al., 2000, Envir. Health Persp. 108:21-27). The chain growth polymerization mechanism results in long chains and therefore early gelation which contributes to both volume shrinkage and shrinkage stress. The current systems typically only reach a final double bond conversion of 55 to 75%, which not only contributes to the insufficient wear resistance and mechanical properties, but also jeopardizes the biocompatibility of the composites due to the leachable unreacted monomers. Additionally, the residual monomer left in the restoration after curing is extractable and may leach out of the restoration and into the body, with unknown consequences (Sasaki et al., 2005, J. Mat. Sci.: Mat. Med. 16:297-300; Pulgar et al., 2000, Envir. Health Persp. 108:21-27). There is concern that residual monomers may cause allergic reactions and sensitization in patients (Theilig et al., 2000, J. Biomed. Mat. Res. 53(6):632-639). There is also reason to believe that release of the most common reactive diluent, triethylene glycol dimethacrylate (TEGDMA), may also contribute to local and systemic adverse effects by dental composites (Hansel et al., 1998, J. Dent. Res. 77(1):60-67; Englemann et al., 2001, J. Dent. Res. 80(3):869-875; Schweikl and Schmalz, 1999, Mut. Res.-Gen. Toxic. Envir. Mutag. 438:71-78; Darmani and Al-Hiyasat, 2006, Dent. Mat. 22:353-358).
Upon polymerization, shrinkage stresses transferred to the tooth can cause deformation of the cusp or enamel microcracks (Davidson and Feilzer, 1997, J. Dent. Res. 25:435-440; Suliman et al., 1993, J. Dent. Res. 72(11):1532-1536; Suliman et al., 1993, J. Dent. Res. 9(1):6-10), and stress at the tooth-composite interface may cause adhesive failure, initiation of microleakage and recurrent caries. In addition, significant increases in volumetric shrinkage and shrinkage stress are experienced when the double bond conversion is increased to reduce the leachable monomer (Lu et al., 2004, J. Biomed. Mat. Res. Part B—Applied Biomat. 71B:206-213). This trade-off of conversion and shrinkage has been an inherent problem with composite restorative materials since their inception.
Recently, thiol-enes have been investigated as alternatives to dimethacrylate dental restorative materials (Lu et al., 2005, Dent. Mat., 21(12):1129-1136). The reactions proceed via a step growth addition mechanism that comprises the addition of a thiyl radical through a vinyl functional group and subsequent chain transfer to a thiol, regenerating the thiyl radical (Jacobine, A. F. Radiation Curing in Polymer Science and Technology III, Polymerisation Mechanisms; Fouassier, J. D.; Rabek, J. F., Ed.; Elsevier Applied Science, London, 1993; Chapter 7, 219; Hoyle et al., 2004, J. Pol. Sci.: Part A: Pol. Chem. 42:5301-5338; Cramer and Bowman, 2001, J. Pol. Sci. Part A. Pol. Chem. 39(19):3311; Cramer et al., 2003, Macromol. 36(12):4631; Cramer et al., 2003, Macromol. 36(21):7964; Reddy et al., 2006, Macromol. 39(10):3673). The step-growth polymerization mechanism results in shorter polymer chains and delayed gelation, resulting in reduced volume shrinkage and shrinkage stress. It is well known that in thiol-ene step growth polymerizations, the thiol and ene components must be present in a 1:1 stoichiometric ratio of functional groups to achieve complete conversion and maximize polymer mechanical properties (Morgan et al., 1977, J. Polym. Sci. A, Polym. Chem. 627; Jacobine et al., 1992, J. Appl. Pol. Sci. 45(3):471-485; Cramer and Bowman, 2001, J. Pol. Sci. Part A. Pol. Chem. 39(19):3311; Hoyle et al., 2004, J. Pol. Sci.: Part A: Pol. Chem. 42:5301-5338). The high functional group conversion of thiol-ene polymers significantly mitigates the problems associated with current dimethacrylate resin systems which are associated with incomplete double bond conversion. Besides the impact of the polymerization mechanism on the gel point conversion and network formation, the thiol-ene systems have advantageous curing kinetics demonstrating rapid polymerization rates, high overall functional group conversion, and little sensitivity to oxygen inhibition (Lu et al., 2005, Dent. Mat. 21(12):1129-1136; Cramer et al., 2002, Macromol. 35:5361; Hoyle et al., 2004, J. Pol. Sci.: Part A: Pol. Chem. 42:5301-5338).
Most importantly for dental restorative materials, thiol-enes exhibit reduced shrinkage and shrinkage stress due to the step growth mechanism and delayed gel point conversion (Chiou et al., 1997, Macromol. 30:7322; Lu et al., 2005, Dent. Mat., 21(12):1129-1136). As a result of the delayed gel point, much of the shrinkage occurs before gelation, which dramatically reduces the shrinkage stress in the final polymer material.
The thiol-ene polymerization has also demonstrated thicker curing depth than methacrylate based resin systems. This can reduce the patient's chair-time since one-step curing is feasible, especially for large cavity filling, where incremental filling has to be applied using current dental composite systems. In addition, the thick cure depth and lack of oxygen inhibition of thiol-ene systems leads to fewer filling and curing steps during restorations, compared with the incremental filling technique using current dimethacrylate dental resin systems.
Unfortunately, despite several advantages of the thiol-ene resin systems for use as dental restorative materials, previous studies have also shown that traditional binary thiol-ene systems exhibit mechanical properties that are not ideal; specifically low flexural modulus and strength relative to dimethacrylate resins (Lu et al., 2005, Dent. Mat., 21(12):1129-1136).
Previous experiments utilizing methacrylate-thiol and acrylate-thiol systems have shown that methacrylate and acrylate functional groups are preferentially consumed due to their participation in both step and chain growth addition reactions (Cramer and Bowman, 2001, J. Pol. Sci. Part A. Pol. Chem. 39(19):3311; Lee et al., Macromol. 40(5), 1466, 2007; Lecamp et al., Polymer 2001, 42, 2727). However, to date, only 1:1 thiol-ene stoichiometry has been investigated in acrylate-thiol-ene and methacrylate-thiol-ene systems (Senyurt et al., 2007, Macromol. 40(14):4901-4909; Wei et al., 2007, J. Pol. Sci. Part A-Pol. Chem. 45(5):822-829; Lee et al., 2007, Macromol. 40(5):1466). Low ene conversion has been reported in these systems in the cases where both (meth)acrylate and ene functional group conversions have been resolved in FTIR (Lee et al., 2007, Macromol. 40(5):1466; Cramer et al., 2010, Dent. Mater. 26(1):21-28).
There is thus a need in the art for novel rapidly curing dental restorative materials with improved monomer conversion and mechanical properties. Such materials should present reduced volumetric shrinkage and shrinkage stress. The present invention fulfills these needs.